1. Field of the Invention
This invention relates to new, storage stable isocyanatocarboxylic acids wherein the isocyanate groups are attached to tertiary carbon atoms and carboxyl groups which may be at least partially present in carboxylate salt form, to a process for their preparation and to their use as cross-linking agents for aqueous coating compositions.
2. Description of the Prior Art
The importance of aqueous lacquers and coating compositions has increased in recent years due to the ever stricter emission guidelines concerning solvents released in the application of lacquers. Although aqueous coating compositions are now available for many fields of application, in many cases they can only attain the high level of quality of conventional, solvent-containing coating compositions with regard to their resistance to solvents and chemicals and to mechanical attack if they are subjected to a chemical cross-linking during film formation.
Only a few attempts have previously been made to achieve a cross-linking of aqueous coating compositions by means of compounds containing free isocyanate groups, i.e., to apply the concept of reactive polyurethane coating compositions which has proved satisfactory in the field of solvent-containing coating compositions to aqueous systems.
DE-OS 2,708,442 describes the addition of monomeric organic diisocyanates for improving the properties of aqueous polyurethane dispersions.
According to DE-OS 3,529,249 organic polyisocyanates may be used for increasing the resistance to solvents and chemicals and improving the wear resistance of coatings based on homo and copolymers dispersed in water. The described positive effects on the coating properties may be attributed to the formation of a "shell of urea" around the dispersed polymer particles resulting from a reaction of the isocyanates with water. Therefore, the polyisocyanates used as additives do not act as cross-linking agents for aqueous dispersions of synthetic resins or synthetic resin precursors containing isocyanate reactive groups.
The preparation of an aqueous polyurethane reactive system from selected aqueous polyhydroxyl compounds which have an emulsifying action and low viscosity free polyisocyanates is the disclosed in DE-OS 3,829,587. The properties of the coatings obtained are equal to those of conventional solvent-containing, two-component lacquers. The process is, however, limited to special polyol dispersions due to the generally poor dispersibility of organic polyisocyanates in water. To apply this method to any aqueous binders containing isocyanate reactive groups would require hydrophilically modified, self-dispersible polyisocyanate components.
Aromatic, hydrophilically modified polyisocyanates such as those described, e.g., in DE-OS 2,359,613 and EP-A-61,628, are not suitable for use in aqueous coating compositions due to their high reactivity with water and consequent evolution of carbon dioxide. They are preferentially used for the production of foams and in the field of adhesives. Isocyanate-functional, cross-linking agents for aqueous coating systems can only be prepared using less reactive (cyclo)aliphatic polyisocyanates.
Hydrophilically modified aliphatic polyisocyanates are described as additives for aqueous adhesives in EPA-0,206,059. These polyisocyanates are rendered emulsifiable by the incorporation of polyether chains containing ethylene oxide units. Such hydrophilicised polyisocyanates are, however, less suitable for use in the field of lacquers because the coatings remain hydrophilic on account of the relatively high polyether content.
Polyisocyanate mixtures containing carboxyl groups and containing uretdione groups as an essential feature of the invention are described in DE-OS 4,001,783. These products are used inter alia as binders for coating compositions. Since preparation of the polyisocyanates containing uretdione groups requires a separate process step, namely the dimerization of the starting isocyanates, this is a relatively complicated process. Another disadvantage common to basically all processes for the preparation of products containing isocyanate groups from the diisocyanates known in polyurethane chemistry, such as hexamethylene diisocyanate and isophorone diisocyanate, is that after these products have been prepared, they have a residual monomer content which is too high for the required standards of work hygiene and must subsequently be removed by expensive processes, e.g., thin layer distillation.
Therefore, it is an object of the present invention to provide compounds which 1) contain both isocyanate groups and carboxyl groups, 2) are storage stable, i.e., undergo no significant amount of reaction between the isocyanate groups during storage, 3) are soluble or dispersible in water after neutralization with bases and 4) have a pot life of several hours in the aqueous phase and are suitable for cross-linking aqueous coating compositions. Further, the process for the preparation of these compounds should result in products containing less than 2% by weight of monomeric diisocyanates without requiring an expensive after-treatment.
These objects may be achieved with the isocyanatocarboxylic acids according to the present invention which are described below and the process for their preparation.